Continued worldwide demand for petroleum products, combined with a high level of prices for petroleum and products recovered therefrom, has sustained interest in hydrocarbon sources which are less accessible than crude oil of the Middle East and other geographic regions. Such hydrocarbonaceous deposits range from heavy oil to tar sands, found in western Canada and in the western United States. Depending on the type and depth of the deposit, recovery techniques range from steam injection to in-situ combustion to mining.
For heavy oils in the gravity range of 10 to 20 degrees API, steam injection has been a widely applied method for oil o recovery. Problems arise, however, when attempting to apply this process to subterranean oil reservoirs which even though are relatively porous and contain a significant proportion of oil, are so impermeable as to be productive of substantially no fluid in response to a conventional steam drive application. Such a reservoir is typified by the diatomite formations in the Lost Hills or Cymric Fields which are characterized by depths of about 1000 feet, with thicknesses of about 100 to 300 feet; and having a porosity of about 50%, an oil saturation of about 60%, an oil API gravity between about 13 to 30 degrees, a water saturation of about 40%, and a matrix permeability of less than about 1 millidarcy. These heavy oil formations have been found to yield only a small percentage of their oil content, such as 1% or less, in primary production processes; and have been substantially nonresponsive to conventional types of secondary or tertiary recovery processes.
The literature has seen many attempts aimed at recovering Oil from substantially impermeable types of subterranean formations, such as diatomite, through the use of steam injections techniques. One such method is found in U.S. Pat. No. 4,828,031 to Davis, and assigned to the assignee of the present invention. The method involves the injection of a solvent into the diatomite, followed by injection of a surface active aqueous solution containing a diatomite/ oil-water wettability improving agent, along with a surface tension lowering agent to enhance oil recovery during steam injection.
Another method taught in U.S. Pat. No. 5,085,276 to Rivas, also assigned to the assignee of the present application and incorporated specifically herein by reference, utilizes a series of short steaming cycles at sufficient pressure to induce fracturing of the adjacent formation; alternating with a production cycle which exploits the flashing of the heated formation water from a liquid to steam as wellbore pressures decrease during the transition from the injection to the production cycle. Because the low permeability and high oil viscosity characteristics associated with heavy oil diatomite formations precludes the use of conventional steam stimulation or drive processes, the Rivas method of alternating short steaming and production cycles is effective in recovering hydrocarbons from low permeability formations such as a diatomite matrix. However, the Rivas method, being a single well process, is limited to an operational area heated during the steam injection cycle, that area being adjacent to and surrounding the fractures extending from the wellbore; necessitating a large number of wells to process a given area since each single well will only recover a fraction of the original oil in place because of each well's limitation of only contacting and heating a small area away from the fracture due to the formation's extremely low permeability.
What is needed, therefore, is a steam drive method applicable to formations having low permeability and high oil viscosity, such as heavy oil diatomite formations, but not having the prohibitively large production response time inherent in conventional steam drive operations applied to such low permeability matrixes.